To generate the vast repertoire of antigen receptors required for effective immune recognition, vertebrate animals rely on a unique process known as somatic DNA recombination. During lymphocyte development, gene segments that are separated in germline DNA are juxtaposed with each other to form functional genes. As the genes are assembled randomly in individual cells, an immense repertoire is created from a limited set of germline encoded gene segments. The same recombinase machinery is apparently used by both T and B cells to assemble the genes encoding the variable portion of the T cell receptor or immunoglobulin chains, respectively. Although the expression of the recombinase machinery and the accessibility of the individual DNA targets are regulated independently, the coordinated result is the rearrangement of each locus is generally limited to the appropriate lineage at the appropriate stage. One of the fundamental questions concerning the control of somatic recombination is how DNA sequences, and the proteins that bind them, can dictate the precise tissue and stage specific activation or inactivation of individual loci. Evidence gathered to date strongly suggests that DNA elements initially described as controllers of gene transcription, such as enhancers and promoters, can mediate an additional function as controllers of DNA recombination. The specific aims described below serve to direct experiments into the role of transcriptional promoters in targeting TCR genes for rearrangement at distinct stages of thymocyte development. The following specific questions will be addressed: (1) How does a Vb promoter region contribute to the tissue and stage specificity of Vb recombination? (2) What DNA sequences define the Db promoter, and how does this element contribute to b chain rearrangement? (3) What are the cis-acting elements and trans-acting factors responsible for the developmentally regulated activation of the Ja germline promoter located in the "T early alpha" (TEA) region? Can this v locus promoter confer a distinct pattern of rearrangement to Vb or Db gene segments? To address these questions, experiments will make use of both transgenic mice and tissue culture systems. The long-term goals are to determine whether the trans-acting factors and cis-acting elements that dictate promoter dependent transcription are distinct from, the same as, or partially overlapping with those that control recombination. These data will have profound implications for understanding how promoters and enhancers can act in concert to regulate large chromosomal regions in a tissue and stage specific manner.